Apparatuses which need to have the function of detecting the location of a metal body are, for example, metal detectors and game machines. By way of example, some of the game machines are such that a metal body, e.g., a metal ball is moved within a specified space which has been set in the game machine, and that whether or not a prize is won is determined in accordance with the movement of the ball. A typical example of such a game machine is, for example, a "pachinko" (Japanese upright pinball) game machine with which a game player causes a metal "pachinko" ball to move down within a space held between parallel planes and provided with a large number of obstacles.
The "pachinko" game machine has a panel which defines the space for moving the "pachinko" ball, a glass plate which covers the panel at a fixed interval therefrom, and a projectile mechanism which functions to project the "pachinko" ball to the upper part of the panel. The "pachinko" game machine is so installed that the panel extends substantially in the vertical direction. The panel is formed with a plurality of safe holes, each of which serves to make a hit when the "pachinko" ball has been led thereinto and driven out of the panel, and a single out hole into which the "pachinko" balls having failed to enter the safe holes are finally gathered to be driven out of the panel. Besides, a large number of pins (or nails) are planted on the panel substantially perpendicularly thereto in the stage in which they protrude from the panel to a distance corresponding to the diameter of each "pachinko" ball, in order that the "pachinko" ball falling along the panel may frequently collide against the pins to have its moving direction altered. The pins are arranged on the panel in a predetermined distribution in which, while altering the moving direction of the colliding "pachinko" ball, they lead this ball so as to proceed toward the safe hole in some cases and to miss the safe hole in other cases.
Owing to the construction as stated above, the "pachinko" game machines come to have individualities. A machine may have easy or difficult to register hits depending upon the slight differences of the respective machines in the arrangement and inclinations of the pins. Even identical machines have differences such that it has a high hit rate and a low hit rate for safe hole. Moreover, the differences are various among the machines.
In a game center or the like where the game machines of this type are installed in large numbers, knowing the individualities of the respective game machines is important for management in relation to the profit administration and customer administration of the game center. By way of example, when many of the machines register hits excessively, the game center side suffers a loss, whereas when all the machines are difficult to register hits on, customers become disinterested, which is unfavorable to business. Accordingly, countermeasures need to be taken by knowing the individualities of the respective game machines which are installed in the center.
For such a purpose, it is practiced to detect the moving courses of the "pachinko" balls in the "pachinko" game machine. In the official gazette of Japanese Patent Application Publication No. 3560/1989, for example, there is disclosed an apparatus equipped with an upper sheet and a lower sheet which have a pair of contacts. This technique senses the existence of the "pachinko" ball in such a way that the "pachinko" ball gets on the upper sheet and depresses it, whereby the pair of contacts come into touch.
With the prior-art apparatus, however, since the sheets have the pairs of contacts, they are restricted in arrangement, and they can be arranged only along the passages of the "pachinko" balls. It is therefore impossible to detect the motions of the balls from the point of view at which the whole panel is seen. This results in a difficult problem of detecting, for example, how the balls enter the safe holes and the out hole.
In addition, since the detection is based on the physical touch of the pair of contacts, it can take place in some moving states of the ball that the depression of the sheet becomes too weak to bring the pair of contacts into touch, so the motion of the ball is not detected. Besides, inferior touches can occur due to the wear, corrosion etc. of the pair of contacts. Further, the erroneous touch of the pair of contacts can be incurred by a vibration or the like or by chattering. For these reasons, the apparatus lacks reliability.
Another problem is that, since a pressure applied by the ball is utilized, the motion of the ball is delicately affected.
Such problems can be encountered, not only in the "pachinko" game machine, but also in different machines. It is accordingly desired to overcome these problems.